High temperatures affect the hypersensitive reaction, disease resistance and gene expression induced by a novel harpin HpaG-Xcm

Abstract

Harpin proteins are produced by plant-pathogenic Gram-negative bacteria and regulate bacterial pathogenicity by inducing plant growth and defence responses in non-hosts. HpaG-Xcm, a novel harpin protein, was identified from Xanthomonas citri pv. mangiferaeindicae, which causes bacterial black spot of mango. Here, we describe the predicted structure and functions of HpaG-Xcm and investigate the mechanism of heat resistance. The HpaG-Xcm amino acid sequence contains seven motifs and two α-helices, in the N- and C-terminals, respectively. The N-terminal α-helical region contains two heptads, which form the coiled-coil (CC) structure. The CC region, which is on the surface of HpaG-Xcm, forms oligomeric aggregates by forming hydrophobic interactions between hydrophobic amino acids. Like other harpins, HpaG-Xcm was heat stable, promoted root growth and induced a hypersensitive response (HR) and systemic acquired resistance in non-host plants. Subjecting HpaG-Xcm to high temperatures altered the gene expression induced by HpaG-Xcm in tobacco leaves, probably due to changes in the spatial structure of HpaG-Xcm. Phenotypic tests revealed that the high-temperature treatments reduced the HR and disease resistance induced by HpaG-Xcm but had little effect on growth promotion. These findings indicate that the stability of interactions between CC and plants may be associated with thermal stability of HpaG-Xcm.

The stability of the coiled-coil structure near to N-terminus influence the heat resistance of harpin proteins from Xanthomonas

“…Proteins could lose their functional activities under various stress conditions, mainly due to the destruction of their active three-dimensional (3-D) structure [1]. However, some proteins, including harpins, can maintain their functional activity under heat-stressed conditions [2][3][4]. Harpins are a class of proteins encoded by hrp genes in Gram-negative bacteria and are secreted by the type III secretion system during interactions between pathogens and plants [5].…”

“…Harpins are a class of proteins encoded by hrp genes in Gram-negative bacteria and are secreted by the type III secretion system during interactions between pathogens and plants [5]. The first harpin was originally isolated from Erwinia amylovora [6] and subsequently, nearly two dozen harpins have been found and characterized in the following plant-infecting bacteria: Xanthomonas, Pseudomonas, Erwinia, and Ralstonia [2,4,[6][7][8][9][10][11][12]. To date, most research on harpins has mainly focused on the defining of their roles in plants [3,[13][14][15][16][17].…”

“…Harpins have a common characteristic, heat resistance, which have been reported to maintain their HR-elicitor activity after 10 min (min) treatment at a high temperature of 100°C in tobacco plants [2][3][4]. However, few studies had attempted to elucidate the mechanism responsible for their heat resistance.…”

“…Another one is that a coiled-coil (CC) structure forming at N-terminal α-helical region in HpaXcm from X. citri pv. mangiferaeindicae [2] might attribute to the thermal stability of Hapin. However, the factors affect the heat resistance of harpin protein is still unclear.…”

“…manihotis (Xpm) [34]; and, the bacterial black spot of mango bacteria, X. citri pv. mangiferaeindicae (Xcm) [2]. For the selected proteins, the secondary and spatial structure were predicted, and the ability on HR induction and the helical content in secondary structure were monitored treated with temperatures ranging from 28°C to 200°C.…”